ActualParameters:
	{ActualParameters} '(' (expressions+=Expression (',' expressions+=Expression)*)? ')'
;

MethodCall:
	receiverObject=Expression '.' mixinName=ID '.' methodName=ID params=ActualParameters
;

Expression:
    TerminalExpression ({Expression.expr=current} '.' mixinName=ID '.' memberName=ID actualParams=ActualParameters?)*
;

TerminalExpression returns Expression:
	{ThisExpr} 'this' |
        {NullExpr} 'null' |
        {FalseExpr} 'false' |
        {TrueExpr} 'true ' |
        {StringLiteralExpr} stringLiteral=STRING_LITERAL |
	{NumberLiteralExpr} numberLiteral=NUMBER_LITERAL |
	{IdentifierExpr} identifier=ID |
	{ObjectCreationExpr} objectCreation=ObjectCreation |
        {SuperExpr} 'super' actualParams=ActualParameters |
        {ParenthesizedExpr} '(' parenthesizedExpr=Expression ')'
;

ParamAssignment:
	mixinName=ID '.' paramName=ID ':=' assignedValue=Expression
;

ParamAssignments:
	{ParamAssignments} (paramAssignments+=ParamAssignment (',' paramAssignments+=ParamAssignment)*)?
;

Injector injector = Guice.createInjector(new MyLanguageRuntimeModule());  
Serializer serializer = injector.getInstance(Serializer.class);
MethodCall methodCall = MyLanguageFactory.eINSTANCE.createMethodCall();
methodCall.setReceiverObject(MyLanguageFactory.eINSTANCE.createThisExpr());
methodCall.setMixinName("Point2D");
methodCall.setMethodName("setCoords2D");
ActualParameters params = MyLanguageFactory.eINSTANCE.createActualParameters();
NumberLiteralExpr param1 = MyLanguageFactory.eINSTANCE.createNumberLiteralExpr();
param1.setNumberLiteral("10");
NumberLiteralExpr param2 = MyLanguageFactory.eINSTANCE.createNumberLiteralExpr();
param2.setNumberLiteral("20");

//I'm not sure the underlying code is correct :-/
//how can I add a NumberLiteralExpr to the "expressions" collection of a ActualParameters object?

params.getExpressions().add(param1);
params.getExpressions().add(param2);

methodCall.setParams(params);
System.out.println(serializer.serialize(methodCall));

org.eclipse.xtext.parsetree.reconstr.XtextSerializationException: Serialization failed
<# of serialized tokens>: <EObject path> "<serializable fragment, starting from the end>":
  -> <possible reasons for not continuing>
13:MethodCall.params->ActualParameters: "( 20 )":
  -> ActualParameters_ActualParametersAction_0: n/a
11:MethodCall.params->ActualParameters.expressions[1]->NumberLiteralExpr: ")":
  -> TerminalExpression_NumberLiteralAssignment_5_1: n/a
5:MethodCall.params->ActualParameters: "( )":
  -> ActualParameters_ActualParametersAction_0: n/a